The present invention relates to a combination of materials for the production of mercury-dispensing devices, to the mercury-dispensing devices thus produced and to a process for the introduction of mercury inside electron tubes.
The use of small amounts of mercury in electron tubes such as, for example, mercury-arc rectifiers, lasers, various kinds of alphanumeric displays and, particularly, fluorescent lamps is well known in the art.
A precise dosage of mercury inside these devices is extremely important for the quality of the devices and most of all for ecological reasons. In fact, the high toxicity of this element implies serious problems of ecological nature upon end-life disposal of the devices containing it, or in case of accidental break-up of the devices. These problems of ecological nature impose the use of amounts of mercury as small as possible, compatibly with the functionality of the tubes. These considerations have been lately included also in the legislative sphere, and the trend of the recent international regulations is to establish upper limits for the amount of mercury which can be introduced into the devices: for example, for standard fluorescent lamps the use of a total amount of mercury (Hg) not greater than 10 milligram (mg) per lamp has been suggested.
In the past mercury was introduced into the tubes in liquid form. However, the use of liquid mercury poses problems concerning the storing and handling in the plants for the production of tubes, due to its high vapor pressure also at room temperature. Secondly, a common drawback of the techniques for the introduction of mercury into the tubes in liquid form is the difficulty in precisely and reproducibly dosing volumes of mercury on the order of microliters, which difficulty usually leads to the introduction of amounts of the element in amount much higher than needed.
These drawbacks have lead to the development of various alternatives to the use of liquid mercury in free form.
The use of liquid mercury contained in capsules is disclosed in several documents. This method is described, for example, in U.S. Pat. Nos. 4,823,047 and 4,754.193, referring to the use of metallic capsules, and in U.S. Pat. Nos. 4,182,971 and 4,278,908 wherein the mercury container is made of glass. After closing the tube, the mercury is released by means of a heat treatment which causes the breakage of the container. These methods generally have some drawbacks. First of all, the production of the capsules and their mounting inside the tubes may be complicated, especially when they have to be introduced inside small-size tubes. Secondly, the breakage of the capsule, particularly if it is made of glass, may produce fragments of material which can jeopardize the tube quality, so much so that U.S. Pat. No. 4,335326 discloses an assembly wherein the mercury-containing capsule is in turn located inside a capsule acting as a shield for the fragments. Moreover, the release of the mercury is often violent, with possible damages to the inner structure of the tube. Finally, these systems still have the drawback of employing liquid mercury, and therefore they do not completely solve the problem of the precise and reproducible dosage of few milligrams of mercury.
U.S. Pat. No. 4,808,136 and the European patent application EP-568,317 disclose the use of tablets or small spheres of porous material soaked with mercury which is released by heating. However, these methods also require complicated operations for the loading of mercury into the tablets, and the released amount of mercury is difficult to reproduce.
These problems are overcome by U.S. Pat. No. 3,657,589 assignee of the present invention, which discloses the use of intermetallic compounds of mercury having the general formula Ti.sub.x Zr.sub.y Hg.sub.z, wherein x and y may vary between 0 and 13, the sum (x+y) may vary between 3 and 13 and z may be 1 or 2.
These compounds have a temperature of mercury-release start variable according to the specific compound, however they are all stable up to about 500.degree. C. both in the atmosphere and in evacuated volumes, thus being compatible with the operations for the assembly of the electron tubes, during which the mercury-dispensing devices may reach temperatures of about 400.degree. C. After closing the tube, the mercury is released from the above-cited compounds by an activation operation, which is usually carried out by heating the material between 750.degree. C. and 900.degree. C. for about 30 seconds. This heating may be accomplished by laser radiation, or by induction heating of the metallic support of the Hg-dispensing compound. The use of the Ti.sub.3 Hg compound, manufactured and sold by the assignee of two present invention under the trade name St505 is particularly advantageous; in particular, the St505 compound in sold in the form of compressed powder in a ring-shaped container or of compressed powder in pills or tablets, under the trademark "STAHGSORB", or in the form of powders laminated on a metallic strip, under the trademark "GEMEDIS".
These materials offer various advantages with respect to the prior art:
as mentioned above, they avoid the risks of mercury evaporation during the cycle of production of the tubes, in which temperatures of about 350.degree.-400.degree. C. may be reached; PA1 as described in the cited U.S. Pat. No. 3,657,589, a getter material can be easily added to the mercury-dispensing compound with the purpose of chemisorption of gases such as CO, CO.sub.2, O.sub.2, H.sub.2 and H.sub.2 O, which would interfere with the tube operation; the getter being activated during the same heat treatment for the release of mercury; PA1 the released amount of mercury is easily controllable and reproducible. PA1 a mercury-dispensing intermetallic compound A including mercury and a second metal selected among titanium, zirconium and mixtures thereof; PA1 an alloy or an intermetallic compound B including copper, a second metal selected among tin, indium, silver or combinations thereof, and possibly a third metal selected among the transition elements.
Despite their good chemical-physical characteristics and their great ease of use, these materials have the drawback that the contained mercury is not completely released during the activation treatment. In fact, the processes for the production of mercury-containing electron tubes include a tube-closing operation performed by glass fusion (e.g. the sealing of fluorescent lamps) or by frit sealing, i.e. welding two pre-shaped glass members by means of a paste of low-melting glass. During these operations, the mercury-dispensing device may undergo an indirect heating up to about 350.degree.-400.degree. C.; in this step the device is exposed to gases and vapours emitted by the melted glass and, in almost all industrial processes, to air. In these conditions, the mercury-dispensing material undergoes a surface oxidation, whose final result is a yield of about 40% of the total mercury content during the activation process.
The mercury not released during the activation operation is then slowly released during the life of the electron tube.
This characteristic, together with the fact that the tube must obviously work from the beginning of its life cycle, leads to the necessity of introducing into the device an amount of mercury which is about double than that which would theoretically be necessary.
In order to overcome these problems, patent application EP-A-091,297 suggests the addition of Ni or Cu powders to the Ti.sub.3 Hg or Zr.sub.3 Hg compounds. According to this document, the addition of Ni and Cu to the mercury-dispensing compounds causes the melting of the combination of materials thus obtained, favouring the release of almost all the mercury in few seconds. The melting takes place at the eutectic temperatures of the systems Ni--Ti, Ni--Zr, Cu--Ti and Cu--Zr, ranging from about 880.degree. C. for the Cu 66%-Ti 34% composition to 1280.degree. C. for the Ni81%-Ti 19% composition (atomic percent), though the document erroneously gives a melting temperature of 770.degree. C. for the Ni 4%-Ti 96% composition. The document acknowledges that the mercury-containing compound is altered during the tube working treatments, and it needs a protection; to this purpose, there is suggested to close the powder container by means of a steel, copper or nickel sheet which is broken during the activation by the pressure of the mercury vapor generated inside the container. This solution is not completely satisfactory: in fact, as in the methods employing capsules, the mercury bursts out violently and can cause damages to portions of the tube. In addition, the manufacturing of the container is quite complicated, since it requires the welding of small-size metallic members. Furthermore, this document does not contain experimental data to support the assessed good mercury-release characteristics of the combinations indicated.